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Possibly related info: Lenovo Legion-Y540. Windows was installed first. Ubuntu couldn't detect any other OS while installing using the .iso on a bootable USB device too.

Firstly, I literally have no idea about how Operating Systems work at lower levels and anything related to them. Plus, I have almost no experience in any sort of UNIX system. I have installed Ubuntu on my computer on a particular partition on my SSD, assuming that I could dual boot. Now I have this problem:

  • I have only 1 element in Boot Section for UEFI that is not Network related, which starts with "Windows Boot Manager". Fast start-up and secure boot are disabled.

  • If I boot my computer giving UEFI top priority, Windows boots and pressing shift wouldn't trigger GRUB.

  • If I boot my computer enabling Legacy Support and giving it's elements top priority, I can access either to GRUB or to Ubuntu, but on the list there's no Windows.

I need some insight, thanks!

My name is name
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  • If you will reinstall Ubuntu, when booting the usb, be sure that the entry in the bios/uefi boot menu reads `UEFI: your usb` (if that is availble). In orther to install Ubuntu in uefi mode mode, you have to boot the installer in uefi mode. – schrodingerscatcuriosity Mar 08 '20 at 20:14
  • @guillermochamorro if I do not enable Legacy Support, I cannot boot from my USB (USB devices don't appear on BIOS if it's not for Legacy). So I think it's not possible to make it UEFI, right? Maybe I should try looking for other options. – My name is name Mar 08 '20 at 20:33
  • UEFI & Legacy/BIOS are not compatible. And with new systems you want UEFI. But it sounds like you installed Ubuntu in Legacy mode. Boot-Repair can convert it with a total reinstall of grub. But you have to boot live installer in UEFI mode both to repair or to reinstall in UEFI mode. – oldfred Mar 08 '20 at 20:39

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Turns out, the core of the problem was UEFI/Legacy incompatibility. Pre-existing Windows 10 in the drive was installed for UEFI, however, burning the .iso image of the Ubuntu installation in default settings of Rufus would only show my USB media in the Legacy bootable drive list. So I had installed Ubuntu in Legacy Mode, conflicting with Windows that was installed for UEFI. GRUB was not able to see any OS in UEFI mode, and there was only 1 OS installed in Legacy mode which was Ubuntu.

The steps below made my BIOS able to recognize the USB device as a UEFI bootable USB media. So, after being able to disable Legacy Support in the Boot settings, re-installing Ubuntu made everything work like a charm.

While burning the .iso image to the bootable USB(used Rufus);

  1. Chose the partition table for USB media as GPT, as some others suggested to make it match with the partition table of the existing system (Windows 10, UEFI/GPT)

  2. Used a FAT file system with default parameters,

  3. Burned the .iso image to the USB media as a DD image.

My name is name
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  • @Nmath, I have made changes after reading your comment. I think my answer was not that clear so it caused misunderstandings. I believe I made my points more explicit now. Thanks for the answer! – My name is name Mar 09 '20 at 07:31